Support WKProcessPool for cookie sharing

WKWebView provided by the plugin does not use any WKProcessPool. Using WKProcessPool will help in sharing cookies across WKWebView instances. If the cordova app has a usecase where it creates its own WKWebView instance, for instance, a remote login screen, then it is better that the new WKWebView instance uses the same WKProcessPool.

In order to make this happen, I propose creating a WKProcessPool factory in this plugin and publish that, so that the cordova app can use it for their WKWebView instances.

I have a working model in my git branch:
https://github.com/anandvnath/cordova-plugin-wkwebview-engine